Space-current device



Feb. I, 1938. N. c. PEARCY SPACE CURRENT DEVICE Original Filed Feb. 20, 1924 INVENTOR A/a/v 0 Fearcy.

ATT

Patented Feb. 1, 1938 UNITED STATES PATENT OFFICE inghouse Electric &

Manufacturing Company,

a corporation of Pennsylvania Original application February 20, 1924, Serial No.

Divided and this application July 17,

1931, Serial No. 551,411

4 Claims.

This application is a division of application Serial No. 694,028, filed February 20, 1924.

My invention relates to space-current devices and it has particular relation to devices of the above designated character utilizing indirectly heated cathodes for gas filled rectifiers.

One object of my invention is to provide a space-current device having an improved cathode capable of maintaining a large electron emission and which is capable of withstanding the positive-ion bombardment met with in commercial gas-filled rectifiers.

Another object of my invention is to provide a gas filled rectifier that may be efliciently used in connection with large rectified currents.

According to my invention, I provide a gas filled rectifier having a cathode which is either directly or indirectly heated and employs as an electron-emitting element a fused mass of nickel and thermionically active oxides, such as barium oxide, strontium oxide and calcium oxide. I thus obtain an electron-emitting element which has very high emissivity and maintains about the same characteristics for a very long time, far exceeding the life and the performance of the ordinary tungsten electrodes.

My invention is particularly well adapted for use in connection with thermionic tubes of the type described in the copending application of H. M. Freeman and W. G. Wade, Serial No. 611,263, filed January 8, 1923 and assigned to the Westinghouse Electric and Manufacturing Company.

Other objects of my invention consist in the provision of new methods of making an improved space-current device and thermionic cathodes of the above designated character. With the foregoing and other objects in view, my invention consists in the details of structure and methods of construction described hereinafter and illustrated in the accompanying drawing, wherein Figure 1 is a view in elevation of a rectifier according to my invention with a portion of the front wall broken away to show the inner structure.

Fig. 2 is a similar View illustrating a modification of my invention.

Fig. 3 is a sectional view of a thermionic cathode employed in the space-current device shown in Fig. 1.

Fig. 4 is a view, similar to Fig. 3, showing a modified cathode structure.

Fig. 5 is a diagrammatic view of a rectifier system embodying my invention.

By way of illustration, I have shown my oathode applied to a rectifier, although it is susceptible of other uses, as in three-electrode tubes. Referring to Fig. 1, a rectifier comprises an evacuated envelope I having mounted therein an anode 2 and a cathode structure 3 which is supported in a familiar manner by a rod 4 and. a terminal strap 6 of metal constituting the electric connection to the cathode. The envelope is preferably filled with a small quantity of inert gas such as argon or mercury vapor for securing an atmosphere of molecules capable of carrying a discharge between the cathode and the anode at relatively low pressures, as is well known in the art.

The principal feature of my invention is the construction of the cathode 3, one modification of which is shown in Fig. 3 and which comprises a cylindrical member 8 of a refractory insulating material, such as zircon or porcelain, having a heating element 9, of U shape, extending through longitudinally disposed perforations therein. The heating element is supplied with two terminal wires ll, one of which may be connected to the terminal strip 6 leading to the cathode surface. Surrounding the cylindrical member 8 and in close contact therewith is a sleeve l2 comprising a fused mass of metal and oxides of the alkaline earth metals, the sleeve constituting the electron-emitting element 1 of the cathode.

In making the sleeve, I prefer to use nickel on account of its relatively low melting point and since it gives very good performance. As electron-emitting oxides, I employ the oxides of barium, strontium and calcium which are fused or sintered with finely divided nickel particles to form a homogeneous mass and which are firmly held to the nickel, assuring a long life and producing a cathode capable of standing the positive-ion bombardment and maintaining substantially constant performance throughout the life of the rectifier. The cathode just described is much superior to a cathode structure wherein the high electron emissivity is obtained by dipping a metallic surface in a proper solution for obtaining an oxide coat, since the coat which is obtained by such dipping is destroyed, after a relatively short time, by the bombardment of the positive ions and the tube becomes inoperative after a very short period of operation. Another advantage of my improved cathode is that its construction is much cheaper and consumes only a fraction of the time necessary to make an oxide coated electrode by the dipping process.

One of the novel steps of making an improved cathode according to my invention involves the intimate mixture of the finely pulverized compounds of the alkaline earth metals, and also the employment of finely pulverized nickel. To obtain the finely divided nickel I take nickel oxide and heat the same in an atmosphere of hydrogen at approximately 600 C. until the oxide is reduced to metal. To obtain the intimate mixture of compounds of the alkaline earth metals used in my improved method of making the oathode, I dissolve a mixture of barium nitrate, strontium nitrate and calcium nitrate in water and precipitate the carbonates of the above-named metals from the solution so obtained, by adding a solution of ammonium carbonate. The precipitate thus obtained, when filtered, washed and dried, constitutes a very intimate mixture of the V carbonates of the three metals.

The above-described method of obtaining a pulverized mixture of the compounds of the three metals consists, in general, in dissolving soluble salts of the metals which are to be mixed in a suitable solvent and precipitating from the solution insoluble salts of the metals. In a practical instance, I have used the following proportions:

Per cent Barium nitrate 40 Strontium nitrate 26.67 Calcium nitrate -l 33.33

I regard the above-described step of mixing the ingredients constituting the electron-emitting element of the improved cathode as instrumental in securing the performance of the same and desire thatthe same be regarded as a distinct feature of my invention.

The nickel particles and the carbonates of the alkaline earth metals are mixed together and moistened with a suitable binder such as water or an acid, such as acetic acid, for example, which reacts with a portion of the carbonates and forms a soluble salt of the alkaline earth metals. I thus obtain a plastic mass which is extruded through a suitable die into the desired shape. I may form rods or slabs, or in case of a cathode employed in a rectifier as shown in Fig. 1, I form a cylindrical sleeve which fits over the cylindrical refractory element 8.

The extruded body is dried and then heated in an atmosphere of hydrogen to a temperature sufiicient to sinter the materials constituting the same. In a practical instance where I use the proportions of the ingredients named above, I bring the heated mass to a temperature of approximately 1000 C.

After removing the sintered body thus obtained from the furnace, the same is ready to be assembled in the space-current device, the carbonates of the alkaline earth metals forming with the nickel, a homogeneous, mechanically strong mass.

The carbonates may be reduced to oxides either during the sintering process or at any time afterwards, preferably when treating the tube on the pump in a well-known manner, after the same has been completely assembled.

According to another method, I may form the plastic mass of the nickel particles and the carbonates of the alkaline earth metals into a sleeve surrounding the heater member 3 and introduce the composite body thus obtained into the furnace for carrying out the sintering process as described hereinbefore.

The connection of the electric lead or strip 8 to the cathode may be made-either by securing the strip to the cylinder l2 by any mechanical means such as a wire l3 wound around the cylinder or by welding the strap directly to the surface of the cylinder, or by a combination of both methods. The cylinder I2 is secured at its upper end to theholding rod 4, by a strip l4 similar to strip 6 or by any other suitable means;

Rectifiers and tubes made with cathodes obtained in the manner described hereinabove 0perate very successfully and give larger currents at lower voltages than are possible with any other cathode constructionof which I am aware. In particular I am able to operate such cathodes in rectifiers without maintaining .a current in the filamentary heater 9, as the current therethrough may be turned oif as soon as the current flow through the rectifier is initiated, the bombardment of the positive ions being sufficient to maintain the cathode at a temperature where the same is in an electron-emitting state.

An operative unit of the last described character is shown in Fig. 5. A double-wave rectifier is provided with an improved cathode 3a'made according to my invention and two anodes 2a leading to a supply transformer Ila. A directcurrent load device I5 is connected between the active coat l6 of the cathode and the middle terminal 11 of the transformer Ila. The heating filament l8 of the cathode is supplied from an auxiliary transformer winding 19 through a switch 2! which is electromagnetically controlled by the current flowing to the direct current load. The switch is closed when the load current is below a certain value and opened when the current exceeds that value at which the positiveion bombardment maintains the cathode in an electron emitting state.

While a somewhat similar operation of rectifiers has been described heretofore, I am not aware of any practical construction utilizing a cathode which embodied the oxides of the alkaline earth metals and which was capable of producing a rectifier carrying alarge amount of current at the low pressures utilized in my devices. is obtained by means of my improved cathode is many times larger than the electron emission possible when the cathode consists of a meta bead such as tungsten. In Fig. 4, is shown a modification of my invention, the refractory cylinder 8 being coated with nickel or other metal previous to the assembly in the electron-emitting sleeve I2, the whole body being assembled when hot so that the nickel coat is plastic and forms anintimate connection between the sleeve l2 and the refractory member 8, facilitating the flow of heat from the heater member to the sleeve andexpediting the step of initiating the operation of the rectifier.

I have found that the plastic mass of the nickeland alkaline earth metals may be extruded through a suitable die to form a filament similar to that used in carbon filament lamps. Since the oxides of the rare earth metals are of relatively high resistance and are intermingled with the fine metal particles, a filament so obtained is of relatively high resistance .and consumes little current While giving a relatively large electron emission. A filament of this character, when used in a gas-filled tube, is better able to withstand the bombardment of the gas molecules than an ordinary coated filament, and the life of such filament is much longer than can be obtained with other constructions. A tube made according to the last named method is shown in Fig. 2.

For instance, the electron emission which A distinct advantage of the filaments and cathodes made according to my invention is that the same are operated at relatively low temperatures and they are less subject to destruction than cathodes operated at high temperatures.

The directly heated cathodes of the prior art had a disadvantage that is obviated by my indirectly heated cathode construction. In general, these cathodes of the prior art have a long wire for large cathode area with the result that one part of the wire is more negative than another part. In fact, one end of the long cathode could be regarded as negative and the other end positive at any particular instant. The discharge will concentrate on the negative end with the result that there will very likely be hot spots on that end with the consequent damage to the cathode. In my indirectly heated cathode construction, the electron emitting cathode is all one potential so that there is no concentration of the discharge but on the contrary, the high thermal and electrical conductivity of the sintered coating tends to evenly distribute the discharge over the cathode surface.

My invention also provides a practical construction for larger currents than have hitherto been practicable with the gas rectifier, having directly heated cathodes. Large currents require a large cathode surface and a high temperature. With the directly heated cathodes of the prior art, these requirements meant a small diameter wire in order to provide the high temperatureand an extremely long one to provide the large surface. However, the practical difficulties of supporting such a long wire and maintaining the slender wire under bombardment have limited the size of the prior gas filled rectifiers.

Another diificulty is that large currents under very small voltage pressure are very unstable and diflicult to regulate in a long wire subject to a discharge.

With my construction of an indirectly heated cathode, however, the electron emitting surface and the connections thereto can be designed most efficiently for the discharge or rectified current and the heater circuit can be designed for the most efficient supply of heat. If a small diameter heater wire is used, it is protected from the discharge by the refractory insulating material. The size of this refraction core makes the electron emitting surface surrounding it inherently large.

I do not desire to be limited to the precise process and details of construction described hereinbefore, but I desire that the appended claims shall be given the broadest construction consistent with the prior art.

I claim as my invention:

1. A luminous electrical discharge tube enclosing a gaseous atmosphere of such a pressure that gas ions carry a substantial part of the current, and comprising an anode and a thermionic cathode comprising a heating element and a refractory insulating member in good thermal relation therewith, said insulating member having a coating comprising a sintered mixture of metal particles and thermionically active particles.

2. A luminous electrical discharge tube enclosing a gaseous atmosphere of such pressure that the discharge is substantially not a pure electron discharge, and comprising an anode and a thermionic cathode comprising a heating element and a refractory insulating member in good thermal relation therewith, said insulating member having a coating comprising a sintered mixture of metal particles and thermionically active particles.

3. A commercial rectifier for carrying a large amount of current comprising a vacuum-tight enclosure containing a cathode and at least one cooperating electrode, said cathode having a unipotential surface adapted to emit electrons freely when maintained at an elevated temperature, said cathode having a surface coating comprising a sintered mixture of metal particles and thermionically-active particles, the electronemissive area of said surface being sufiicient for a current of large commercial size, an insulating supporting member for said surface, an electric heater for said surface separated therefrom by said insulating member, and a gaseous atmosphere of substantial pressure adapted to conduct large currents at low voltage.

4. A commercial rectifier for carrying a large amount of current comprising a vacuum-tight enclosure containing a cathode and at least one cooperating electrode, said cathode having a unipotential surface adapted to emit electrons freely when maintained at an elevated temperature, said cathode having a surface coating comprising a sintered mixture of nickel particles and thermionically-active particles, the electron-emissive area of said surface being sufficient for a current of large commercial size, an insulating supporting member for said surface, an electric heater for said surface separated therefrom by said in-' sulating member, and a gaseous atmosphere of substantial pressure adapted to conduct large currents at low voltage.

NOAH C. PEARCY. 

